1. Field of the Invention
The invention relates generally to method and apparatus for water content determination and particularly to computer-controlled microwave drying of material samples with precision and rapidity.
2. Description of the Prior Art
Moisture content determinations have conventionally been made through the use of ovens utilizing electrical resistance or gas heating. Use of conventional ovens for moisture determination require drying of a specimen of material over a specified period of time with a mass determination taken both before and after the drying period. Water content is usually taken to be the ratio of the mass of water to the mass of dry sample expressed as a percentage. Determination of water content according to the prior art thus described is certainly simple and accurate but it is also extremely slow. In construction situations where large quantities of soil are being placed and compacted, such as in the construction of embankment dams or large foundations, water content determinations of soils must be regularly taken since water content and density of soils dictate the strength characteristics and behavior of the soil. A soil structure compacted to an incorrect water content will experience performance and maintenance problems. However, in most construction situations it is not possible to wait up to 24 hours for a correct water content determination in a conventional oven. Accordingly, method and apparatus are required in such situations in order that a rapid and accurate water content determination can be performed. Other industrial situations wherein the availability of a rapid and accurate water content determination would be of benefit include the manufacture of paper pulp and wood pulp among others and also including the testing of seed, chemicals, fiber, etc. Although methodology exists for water content determination in a more rapid manner than is possible with conventional ovens, this prior methodology is not sufficiently accurate for the requirements of earthwork compaction control as well as in many other water content testing situations.
As examples of prior art which relates to testing for water content of a specimen, attention is directed to Preston U.S. Pat. No. 4,406,070 who discloses a method and apparatus for treating variable batches of moist material by the application of radio frequency energy in order to reduce the moisture content of the material. Radio frequency power brought to bear on the material is incrementally reduced by an electromechanical adjusting device.
Doll U.S. Pat. No. 4,466,198 describes the removal of moisture from wood by subjecting the wood to dielectric heating.
Ellingson, in U.S. Pat. No. 4,483,082, discloses the drying of fabric through microprocessor control. Russian patent 983,416 discloses a process of drying material by using electromagnetic oscillations. An initial moisture content and weight are fed to the memory of a microprocessor in the system described in the Russian patent, the microprocessor through an actuator switching on an electromagnetic oscillation to initiate the drying process. Canadian patent 1,068,515 describes method and apparatus for determining the weight and moisture content of materials disposed within a shipping container. A publication by Diprose et al resulting from the Fourteenth Microwave Power Symposium, June 11-15, 1979, in Monaco, is entitled "The Measurement of Soil and Leaf Moisture Content by 2450 MHz Radiation" and is located on pages 137-140 of the proceedings of that Symposium The Diprose et al publication discloses a process of using microwave radiation for soil drying. A soil sample is weighed, dried and then reweighed according to Diprose et al in order to determine the moisture content of the soil.
Although the Diprose et al article teaches the use of microwave radiation for drying a material and the Russian patent discloses a microprocessor control for a material drying process, the prior art noted above does not disclose a drying apparatus utilizing a computer to monitor weight loss during microwave drying operations and further to adjust microwave power according to conditions existing as drying progresses.
Accordingly, the prior art has not met the long felt need in the art for a rapid, precise and reproducible method for determining water content in a specimen and especially to an accuracy which would be equivalent to that accomplished in a conventional, constant oven maintained at 110.degree. C., for example, and which is sufficiently rapid so as to be utilizable on site for proper earthwork compaction control during a construction process.